In all fiber lines some kind of washing means is incorporated in order to separate the liquor from the digestion of the pulp. Later in the process washing means are then provided in order to separate bleaching liquor after the bleaching steps. There are a number of different types of washing equipment each of which works according to different principles.
A well known washing apparatus is the drum washer, wherein the pulp is dewatered on a rotating filter drum after the addition of washing liquid, and which displaces the liquor remaining in the liquor after previous process steps, e.g. a digestion step or a bleaching step. A negative pressure inside the drum makes the displaced liquid pass through a perforated plate arranged on the rotating drum. One development of the original drum washer is the pressurized displacement washer, wherein the filtrate under a positive pressure is made to pass through the plate. The increased pressure difference brings about a more effective displacement of the filtrate.
According to a previously known construction of a pressurized displacement washer the drum is provided with compartments, which extend in the axial direction of the drum, and are intended to be filled with pulp. The compartments are defined by walls in the form of strips provided axially all along the axis of the drum, and a bottom which is comprised of the perforated plate. The compartment spacing of the drum ensures that the cake of the pulp does not crack and move, but instead maintains the formation obtained at the facing. The perforated plate, on which the pulp has settled, is placed at a distance from the main surface of the drum, so that filtrate channels are formed in the space between the drum and the plate. Along the periphery of the drum there are at least as many filtrate channels as there are pulp compartments.
In a drum washer several different washing steps may be performed, with separate addition of washing liquid to the different steps, as well as recirculation of filtrate from one step to be used as washing liquid in another. In order to obtain a maximum washing efficiency the aim is that washing liquid intended for a specific washing step is not moved to a later washing step. (A pressure difference between the steps results in added washing liquid striving to move itself towards the lower pressure.) In order to be able to differentiate between different washing steps, which are performed in one or more washing zones in the drum, and formation steps, which are performed in the formation zone of the drum, and output steps, which are performed in the output zone of the drum (a dry content increasing zone comprises a first part of the output zone), are the respective zones which are sealed with longitudinal (that is axial) seals. These longitudinal seals are placed between the rotating drum and the surrounding casing. The filtrates from the respective zones are separated by seals in a peripherally positioned end valve provided at one or both end covers.
One problem with drum washers of the type having zones separated with the aid of longitudinal seals is that these seals are subjected to abrasion, wear and other stresses. The seals change with time, which has a negative effect on the general washing performance and also leads to the risk of leakage and shutdown.
According to the prior art there is a possibility for the working staff to make manual adjustments of the longitudinal seals. During this positioning of the seals it is thus of great help to obtain continuous and more precise information of the force acting between the drum and the seal during operation.
In Swedish Patent Publication No. 528721 C2, there is disclosed a unit and a method for adjustment of a seal in a washing apparatus for cellulosic pulp, which method comprises the steps of measuring a force acting on a longitudinal seal and moving the seal based on the measured force. It is previously known to arrange a measuring device, such as a load cell, between a jack and a seal, in order to be able to accurately record the force acting against a seal in a direction from the drum, and to then move the seal substantially in the radial direction of the drum, based on the measured force. The jack provides for transmission of a driving force, manually or from a motor, to the seal in order to adjust the seal. In order to avoid the load cell and other components becoming overloaded, a spring package is mounted between the load cell and the seal. The spring package is biased with a predetermined force. If this force is exceeded, the spring package is compressed. The load cell is mounted between the jack and the force transmission shaft. This means that the load cell cannot be exchanged during operation. Furthermore, the jack has to be dismounted in order to be able to exchange the load cell. Furthermore, the force of the spring is different for every spring package of the respective seal because of different tolerances for different seals, which means that according to the construction that is previously known through Swedish Patent Publication No. 528721 C2, it is not possible to determine which bias force is prevailing under operation in the respective spring package.
Thus, there is a demand for an improved solution to the problem of measuring forces acting on at least one seal in a washing apparatus.
One object of the present invention is thus to provide an improved washing apparatus of the type with a rotary drum provided with compartments. More specifically, an object of the present invention is to accomplish a safer and more efficient sealing mechanism for the washing apparatus. A further object hereof is to accomplish a novel, more cost effective and simpler method for measuring, during operation, the force acting on the longitudinal seal in a radial direction away from the drum. A further object of the present invention is to accomplish a method which makes it possible to exchange measuring means without stopping the operation of the washing apparatus.